Field of the Invention
The invention relates to a pulsed voltage converter and to a method for producing a control value for such a pulsed voltage converter.
A situation often arises in which a DC voltage source is available which supplies a voltage that is other than the voltage which is required. Pulsed DC/DC voltage converters are used to solve this problem, and convert the available voltage to the required voltage. Converters such as these each essentially contain three components, namely a power switch, an energy storage inductor and a smoothing capacitor. These components allow not only step-down converters (output voltage<input voltage) but also step-up converters (output voltage>input voltage) to be produced. The corresponding basic circuits are described in the standard work titled “Halbleiter-Schaltungstechnik” [Semiconductor Circuit Technology] by U. Tietze and Ch. Schenk, 11th Edition (Springer), in Chapter 16.6 (pages 979 et seqq.).
In order to make it possible to produce a controllable DC/DC converter, the power switch must be driven by a square-wave signal whose duty ratio can be readjusted. An analog pulse width modulator, which contains a sawtooth waveform generator and a comparator, may be used to produce such a square-wave signal. The comparator is now supplied first with the triangular waveform voltage produced by the sawtooth waveform generator and second with a control signal that is derived from the output voltage. The comparator switches the switch on for as long as the control signal is greater than the triangular waveform voltage. As soon as the control signal falls below the triangular waveform voltage, the power switch is switched off. This results in a duty ratio that is proportional to the control signal.
A complete implementation of a step-down converter using an analog pulse width modulator is described on pages 983 to 985 of the book titled “Halbleiter-Schaltungstechnik” [Semiconductor Circuit Technology] (11th Edition), as mentioned above, by Tietze/Schenk.
Control circuits that are constructed on an analog basis have the disadvantage that their integration in CMOS circuits is complex, since the analog circuit components are configured from the standardized CMOS circuit design. Furthermore, digital circuits with extensive analog circuit components are known for the large amount of chip surface area that they occupy. For this reason, it would be desirable to be able to replace as great a proportion of the analog circuit components as possible by digital circuits with the same functionality.
U.S. Pat. No. 6,005,377 describes a voltage converter in which the output voltage is digitized by an A/D converter. The digital signal is passed to a comparator, which compares the digital signal with a reference signal in order to produce an error signal. The error signal is filtered and is used for producing a control value for switching time adjustment.
U.S. Pat. No. 5,552,694 discloses a voltage converter, in which the output voltage is intended to be within a limited voltage value range that can be predetermined. If the output voltage is outside the voltage value range, the times for which the control transistors are switched on and off are controlled such that the output voltage returns to within the voltage value interval.